1. Field of the Invention
The present invention relates generally to the encryption of document content, and more particularly, to a method and apparatus for encrypting document content using one-time stencils.
2. Description of Related Art
Cryptography, which includes encryption and decryption, involves the disguising of messages so that only certain people can see through the disguise. Generally, encryption systems perform an encryption operation on a plaintext (i.e., cleartext) message using an encryption key to produce a ciphertext message. The receiver of a ciphertext message performs a corresponding decryption operation with a decryption system using a decryption key to recover the plaintext block. Encryption systems fall into two general categories: symmetric and asymmetric. In the literature symmetric encryption systems are often called secret key systems, and asymmetric encryption key systems are often called public key systems. Symmetric encryption systems use the same secret key (i.e., private key) for both encrypting and decrypting a message, while asymmetric systems use a secret key to decrypt and a public key to encrypt a message.
One example of a symmetric cryptosystem is the Data Encryption Standard (DES) system. In the DES system, an encryption key, which has 56 independently specifiable bits, is used to convert a 64-bit plaintext blocks to ciphertext blocks, or vice versa. Another example of a symmetric cryptosystem is a Boolean one-time pad. The encryption key of a one-time pad is a random string that is the same length as the plaintext input string. Combining the encryption key using bitwise exclusive-OR with the plaintext produces the ciphertext output message. The advantage of one-time pads is that there does not exist a single nontrivial attack when the encryption key is generated completely at random. That is, if the entire encryption key of a one-time pad is random, an attacker with infinite computational resources can only guess the plaintext after seeing the ciphertext.
The operations of such cryptosystems need not be carried out electronically using traditional computational devices such as computers. Alternatively, operations of such cryptosystems can be performed visually, chemically, or by other non-traditional means. For example, U.S. Pat. No. 5,488,664 to Shamir discloses an encryption method that recovers cleartext by stacking one transparency with the encryption key printed on it and another transparency with the cypertext printed on it. This encryption method is a low fidelity one-pass method that is suitable for digital watermarking or the like. In addition, Shamir discloses in Visual Cryptanalysisxe2x80x9d, published at Advances in Cryptology, Eurocrypt. ""98, Springer-Verlag, Berlin, 1998, a high fidelity two-pass method for carrying out parallel computations using photographic films to store the internal state of a bit-sliced computation, and contact printing to perform computational steps of, for example, an exclusive-OR operation.
One advantage of non-traditional cryptographic systems is that they enable cryptographic operations without requiring hardware such as a computer for performing decryption operations. That is, while one party may likely have a computer readily available for performing encryption, the other party decrypting the message may not. Consequently, there continues to be a need to provide non-traditional computational means for decrypting encrypted messages. It would therefore be advantageous to provide a one-time pad encryption system that permits decryption using a conventional printer or copier.
In accordance with the invention, there is provided a method for decrypting original image content in two passes. During a first pass, an encrypted image of the original image content is printed through a stencil to produce a first recording medium with a first partially decrypted image. The stencil blocks portions of the encrypted image from appearing on the first recording medium. During a second pass, a complement of the encrypted image is printed through a complement of the stencil to produce a second recording medium with a second partially decrypted image. The complement of the stencil selectively blocks portions of the complement of the encrypted image from appearing on the second recording medium. The first recording medium is overlaid with the first partially decrypted image on the second recording medium with the second partially decrypted image to fully decrypt the original image content. Alternatively, the original image is fully decrypted by printing the complement of the encrypted image through a complement of the stencil directly onto the first recording medium.